I'm not sure if this is a "light cobalt" or a dark "cornflower" blue, from Russia - or perhaps Belarus. The embossing on the side of the dome looks like C 3 / H (S Z / N) which might be Steklo Zavod HEMAH (Neman Glass Works) or in Cyrillic alphabet, СТЕКЛОЗАВОД HEMAH

HEMAH (Neman) glassworks was formerly the Polish company J. Stolle Niemen, and after the end of WWII and ongoing Soviet expansionism, the portion of Poland along the Niemen River became Byelarussia (White Russia), which is known today as Belarus.

The Polish made predecessorof this insulator is marked JSN for J Stolle Niemen. The Polish term for Glassworks is "HUTY SZKLANE"

Photo courtesy of Pavel Yelkin